The equatorial Atlantic MORBs are derived from a heterogeneous source with 3He/22Ne ra- tios of 6.1 to 9.8 (Table 2.1). The most depleted MORBs are derived from a mantle with the highest3He/22Ne ratios and the more enriched MORBs (Schilling et al., 1994) are de- rived from a mantle with lower3He/22Ne ratios (Fig. 2.2). We note that an average MORB 3He/22Ne
m ratio of 10.2±1.6 was calculated by Honda and McDougall (1998) and8.8±3.5
by Graham (2002). These values were calculated assuming a mantle 20Ne/22Ne ratio of 13.8, and under this assumption, our samples range from 8.9–14.3. The average 3He/22Ne
m ratios
of Honda and McDougall (1998) and Graham (2002) recalculated to 20Ne/22Ne = 12.5 are
7.3±1.2 and 6.1±2.4, respectively, at the mid to low end of our range (Table 2.1; supplemen- tary material).
Figure 2.2: Correlation between3He/22Ne
mand (a) 21Ne/22NeE, (b)εNd whereεis the part in104
deviation from the chondritic 143Nd/144Nd value, and (c)206Pb/204Pb. These correlations establish the
depleted mantle value to be at least 9.8 and indicate that the variability in MORB 3He/22Ne
mratios is
caused by recent mixing between a depleted mantle source and a more enriched plume source with a lower
Since 3He/22Ne
m ratios are strongly correlated with the lithophile isotopic composition
of the MORBs (Fig. 2.2), the variations in 3He/22Ne
m ratios must have been produced by
recent mixing between a depleted mantle with a high3He/22Ne ratio (≥~10) and an en- riched mantle with a low 3He/22Ne ratio. Specifically, intermediate values can be explained by mixing depleted mantle with primitive and recycled material (e.g., Schilling et al., 1994 and Tucker et al., 2012; see supplementary material). Because the lithophile isotopic compo- sition of the depleted samples defines the depleted end of the MORB array (Schilling et al., 1994), the depleted mantle 3He/22Ne ratio must be at least 9.8 (Fig. 2.2). As the3He/22Ne
m
value of 9.8 is itself a lower limit for the most depleted sample (Section 2.2), in subsequent discussions we take the depleted mantle to have a 3He/22Ne ratio of ≥~10.
Previous studies have attributed variations in 3He/22Ne
m ratios in mantle-derived rocks
to either source heterogeneity (Coltice et al., 2011, Kurz et al., 2009, Moreira et al., 2001, Mukhopadhyay, 2012, Raquin and Moreira, 2009 and Yokochi and Marty, 2004) or to frac- tionation resulting from partial melting and degassing of melts derived from a mantle with a uniform3He/22Ne ratio (Hopp and Trieloff, 2008, Moreira and Allègre, 1998, Moreira et al., 2001, Sarda et al., 2000 and Trieloff and Kunz, 2005). However, the correlations between the3He/22Ne
m and lithophile isotopic ratios in the MORB samples (Fig. 2.2) clearly demon-
strate the existence of mantle domains with different3He/22Ne ratios. We further note that the correlations argue against source variations in3He/22Ne arising from fractionation due to fluid/melt partitioning of gases in the deep mantle (Yokochi and Marty, 2004).
2.4 3He/22Ne ratios in the present-day mantle and in planetary materi- als
Based on the mixing trends in Fig. 2.2, the present-day depleted mantle has a3He/22Ne ratio of ≥10. In contrast, plumes tend to have lower 3He/22Ne ratios (e.g. Füri et al., 2010, Gra- ham, 2002, Honda and McDougall, 1998, Jackson et al., 2009, Kurz et al., 2009, Moreira et al., 2001, Mukhopadhyay, 2012, Raquin and Moreira, 2009 and Yokochi and Marty, 2004). OIBs with the most primitive21Ne/22Ne ratios (e.g., Galapagos, Iceland), have 3He/22Ne
Figure 2.3: 3He/22Ne
mratios in modern terrestrial reservoirs along with the3He/22Ne ratios in possible
sources that may have contributed primordial He and Ne to the Earth. Note that the depleted mantle value is at least a factor of 6.5 higher than the value in possible primordial materials. MORBs also have higher values than the primitive reservoir sampled by OIBs at Galapagos and Iceland. 3He/22Ne ratios for terres-
trial reservoirs are computed by Method 1. Details of the calculations are given in the supplementary mate- rial. Data sources are as follows: MORB average (H&M) from Honda and McDougall (1998); MORB aver- age (Graham) from Graham (2002); Mangaia from Parai et al. (2009); Samoa from Jackson et al. (2009); Iceland from Mukhopadhyay (2012); Galapagos from Kurz et al. (2009) and Raquin and Moreira (2009); solar nebula from Grimberg et al. (2006), Mahaffy et al. (1998), and Pepin et al. (2012); implanted solar wind from Raquin and Moreira (2009); chondrite from Ott (2002); iron (Washington County; unclassified) from Becker and Pepin (1984); pallasite (Brenham; olivine) from Mathew and Begemann (1997); howardite (Kapoeta and Jodzie) from Mazor and Anders (1967); aubrites (various) from Lorenzetti et al. (2003); angrite (D’Orbigny) from Busemann et al. (2006). The chondrite, pallasite, howardite, and angrite values are maximum values (see supplementary material).
ratios of 2.3 to 3 (Kurz et al., 2009, Mukhopadhyay, 2012 and Raquin and Moreira, 2009; also see supplementary material).
The present-day mantle is also enriched in 3He relative to 22Ne compared to the sources that may have contributed primordial He and Ne to Earth (Fig. 2.3). These sources include the solar nebula (3He/22Ne =1.46±0.06), the implanted solar (Ne-B) component in gas-rich meteorites (3He/22Ne =0.9±0.1), the chondritic value (3He/22Ne≤0.89±0.11), as well as differentiated meteorites (Fig. 2.3; also see supplementary material).
The particular source(s) of He and Ne to the Earth are unclear, but must be consistent with Ne isotopes. For example, the observation of 20Ne/22Ne ratios >12.9 in the Iceland and Kola plumes (Mukhopadhyay, 2012 and Yokochi and Marty, 2004) implies a nebular origin
for Ne, corresponding to a3He/22Ne of 1.46 ±0.06. The3He/22Ne ratio of the primitive reservoir sampled by plumes is, therefore, fractionated from the nebular value by approxi- mately a factor of 1.5 to 2. The MORB source 20Ne/22Ne ratio of ~12.5 is similar to the Ne- B component observed in some primitive meteorites (Ballentine et al., 2005, Holland and Bal- lentine, 2006 and Raquin et al., 2008), thought to represent implantation of solar wind into dust grains (Grimberg et al., 2006 and Raquin and Moreira, 2009). Alternatively, the MORB source 20Ne/22Ne ratio of ~12.5 could have originated through a limited amount of recycling of atmospheric Ne (20Ne/22Ne = 9.8) into a mantle with nebular Ne (Kendrick et al., 2011). In either case, the 3He/22Ne ratio of the material that contributed He and Ne to the MORB source must have been ≤ 1.46±0.06 (Fig. 2.3). Therefore, the present-day depleted mantle 3He/22Ne ratio is fractionated by at least a factor of 6.5 from possible sources of terrestrial volatiles (Fig. 2.3). Additionally, because of mixing between the depleted mantle and the primitive reservoir over time, the depleted mantle3He/22Ne ratio was likely higher than 10 in the past. Thus, a factor of 6.5 is a lower limit for the degree of 3He/22Ne fractionation of the depleted mantle.
2.5 Can plate tectonics generate reservoirs with high 3He/22Ne ra-